With reference to FIG. 1, a ducted fan gas turbine engine is generally indicated at 10 and has a principal and rotational axis X-X. The engine comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high-pressure compressor 14, combustion equipment 15, a high-pressure turbine 16, an intermediate pressure turbine 17, a low-pressure turbine 18 and a core engine exhaust nozzle 19. A nacelle 21 generally surrounds the engine 10 and defines the intake 11, a bypass duct 22 and a bypass exhaust nozzle 23.
During operation, air entering the intake 11 is accelerated by the fan 12 to produce two air flows: a first air flow A into the intermediate pressure compressor 13 and a second air flow B which passes through the bypass duct 22 to provide propulsive thrust. The intermediate pressure compressor 13 compresses the air flow A directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.
The compressed air exhausted from the high-pressure compressor 14 is directed into the combustion equipment 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines 16, 17, 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines respectively drive the high and intermediate pressure compressors 14, 13 and the fan 12 by suitable interconnecting shafts.
The fan 12 comprises an assembly of blades radially extending from a hub. The fan 12 is surrounded with an annular fan containment casing 20 (having a circular axial cross-sectional profile) for containing a fan blade in the unlikely event of the release of a fan blade from its hub.
This fan containment casing 20 must be capable of withstanding the impact of the released fan blade and must also be able to contain any blade or casing fragments. Furthermore, it must be capable of withstanding the huge loads and vibrations resulting from the out of balance fan blade assembly.
The materials used to construct the fan containment casing 20 are selected for high strength and high ductility. The fan containment casing may consist of either a plain or ribbed metallic casing, for example, formed of ribbed Armco™ or titanium. Other known fan containment casings which were developed to reduce the weight of the fan containment casing comprise a plain or isogrid Kevlar™ wrapped casing e.g. an aluminium isogrid casing wrapped with an aramid fibre weave such as Kevlar™. The Kevlar™ acts to absorb the blade energy by deflecting and stretching thus feeding the load around the casing.
The load results in the circumferential propagation of a transverse displacement wave having a radial amplitude around the annular fan containment casing. Any accessories bolted onto the fan containment casing must be isolated from the Kevlar™ wrapping to ensure that they are not subjected to the transverse displacement wave and remain attached to the fan containment casing.
There is a desire to reduce the propagation of the transverse displacement wave around the circumference of the annular fan containment casing.
Known fan containment casings also typically include a liner bonded to the internal diameter to define a blade tip rub path as well as provide acoustic and aero-elastic functionality. In the event of the release of a fan blade, the liner blunts and turns the trajectory of the released blade so as to impart a glancing blow on the fan case barrel. After the radial loading (resulting in the propagation of the transverse displacement wave around the circumference of the annular fan casing), the fan case assembly is subjected to a torque loading especially in the rare occasion that the following blades pick up on the released blade and drag it around the internal diameter of the casing. This loading is magnified by the rotor out of balance (OOB) which increases both radial and torque loading. The casing has to be thickened (to around 25-30 mm) to resist this loading and maintain its shape without significant delamination or penetration. This increased thickness has undesirable weight and cost implications and most casings will never see these loads in their service lifetime.
There is a desire to increase the resistance to torque loading with a reduced thickness casing and to reduce the OOB interaction.